Effect of whey protein denaturation on structure of casein micelles and their rennetability after ultra-high temperature processing of milk with or without ultrafiltration

Abstract

Abstract Whole milk and skim milk were ultrafiltered to 3× concentration and heated to 72°C, 89°C, 106°C, 123°C, or 140°C in an ultra-high temperature (UHT) plate heat exchanger. Whey protein denaturation increased with heating temperature and was higher in concentrated milk than in the corresponding unconcentrated milk. There was no difference in the patterns of whey protein denaturation and rennet coagulation times between whole milk and skim milk. Rennet coagulation time increased and gel strength decreased with milk processing temperature. Concentrating milk by ultrafiltration shortened coagulation time and increased gel firmness. UHT processed milk did not coagulate when rennet was added. Its 3× concentrated counterpart did coagulate although only a weak gel was formed. UHT heating caused the casein micelles to increase in size with additional protein material adhering to their surface, especially in the 3× skim milk heated to 140°C. This diffuse layer of material around casein micelles was not observed in 3× whole milk. It is suggested that this denatured protein is adsorbed onto the fat-water interfaces during homogenization.